Background of the Invention
1. Field of the Invention
[0001] The present invention relates to an inkjet recording ink composition and an inkjet
recording method.
2. Related Art
[0002] The method for forming an image on a recording medium such as paper based on image
data signals includes an electrophotographic system, a sublimation-type or melting-type
heat-transfer system and an inkjet system. The electrophotographic system requires
a process of forming an electrostatic latent image on a photoreceptor drum through
electrification and exposure and therefore, suffers from complicated system and expensive
apparatus. The heat-transfer system uses an ink ribbon and therefore, suffers from
high running cost and treatment of a waste material, though the apparatus therefor
is inexpensive. The inkjet system uses an inexpensive apparatus and performs the printing
directly on a printing medium by ejecting an ink only on a desired image area, ensuring
efficient use of an ink and low running cost. Furthermore, this system generates less
noises and therefore, is excellent as the image recording system.
[0003] The inkjet recording system includes a system of spraying an ink droplet by utilizing
the pressure of water vapor generated due to heat of a heating element, a system of
spraying an ink droplet by utilizing a mechanical pressure pulse generated from a
piezoelectric element, and a system of spraying an ink droplet containing a charged
particle by utilizing an electrostatic field (see, Japanese Patent No. 3, 315, 334
and U.S.P. 6, 158, 844) . In the system of spraying an ink droplet by water vapor
or mechanical pressure, the spraying direction of an ink droplet cannot be controlled
and due to distortion of an ink nozzle or convection of an air, it is difficult to
land the ink droplet exactly on a desired position of a recording medium.
[0004] On the other hand, the system of utilizing an electrostatic field is an excellent
system, where the spraying direction of an ink droplet is controlled by an electrostatic
field and therefore, the ink droplet can be landed exactly on a desired position,
as a result, a high-quality image-formed material (printed matter) can be obtained.
[0005] The ink composition used in the inkjet recording utilizing an electrostatic field
is an ink composition comprising a dispersion medium and a charged particle containing
at least a coloring material (see, JP-A-8-291267 (the term "JP-A" as used herein means
an "unexamined published Japanese patent application") and U.S.P. 5,952,048). The
ink composition containing a coloring material is useful because four color inks of
yellow, magenta, cyan and black and furthermore, special inks of gold or silver can
be prepared by changing the coloring material and a color image-formed material (printed
matter) can be obtained. However, in the long-term inkjet recording, it has been heretofore
difficult to always stably eject an ink droplet.
Summary of the Invention
[0006] The present invention has been made with an aim to always stably eject an ink droplet
in the inkjet recording for a long period of time and an object of the present invention
is to provide an inkjet recording ink composition and an inkjet recording method,
which can realize the formation of a high-quality image for a long period of time.
[0007] As a result of intensive investigations to attain the above-described object, the
present inventors have found that the ejection property of an ink droplet is greatly
dependent on physical properties of the ink used, requiring these properties each
to fall in a specific range, and also greatly dependent on the method of supplying
the ink to the ejection part. The present invention has been accomplished based on
this finding.
[0008] That is, the present invention has the following constitution.
1. An inkjet recording ink composition comprising:
a dispersion medium; and a charged particle containing a coloring material,
wherein said ink composition satisfies all of the following conditions (A) to
(D):
(A) the electric conductivity at 20°C of the ink composition is from 10 to 300 nS/m,
(B) the electric conductivity at 20°C of the charged particle is 50% or more of the
electric conductivity of the ink composition,
(C) the volume average diameter of the charged particle is from 0.2 to 5.0 µm, and
(D) the viscosity at 20°C of the ink composition is from 0.5 to 5 mPa·s.
2. The inkjet recording ink composition as claimed in the item 1, wherein said charged
particle is dispersed in the dispersion medium by using a graft polymer as a dispersant.
3. The inkjet recording ink composition as claimed in the item 2, wherein said graft
polymer comprises a main chain moiety and a side chain moiety of a graft chain, in
which the main chain moiety is not dissolved in the dispersion medium and the side
chain moiety is dissolved in the dispersion medium.
4. The inkjet recording ink composition as claimed in any one of the items 1 to 3,
wherein the charged particle has an electric charge amount of from 5 to 200 µC/g.
5. The inkjet recording ink composition as claimed in any one of the items 1 to 4,
which comprises the dispersion medium in an amount of from 20 to 99 wt% based on the
entire ink composition.
6. The inkjet recording ink composition as claimed in any one of the items 1 to 5,
wherein the coloring material is a pigment and the pigment content is from 0.1 to
50 wt% based on the entire ink composition.
7. The inkjet recording ink composition as claimed in any one of the items 1 to 6,
wherein the charged particle is a pigment coated with a covering agent.
8. An inkjet recording method comprising spraying, by utilizing an electrostatic field,
an ink droplet of an ink composition comprising a dispersion medium and a charged
particle containing a coloring material, said ink composition satisfying all of the
following conditions (A) to (D),
wherein the method comprises: circulating the ink composition in an ink chamber
having an ejection opening; and applying a voltage to a control electrode in the periphery
of the ejection opening to spray the ink droplet from the tip of an ink guide, in
which the ink guide is present in the ejection opening and the tip of the ink guide
faces a recording medium:
(A) the electric conductivity at 20°C of the ink composition is from 10 to 300 nS/m,
(B) the electric conductivity at 20°C of the charged particle is 50% or more of the
electric conductivity of the ink composition,
(C) the volume average diameter of the charged particle is from 0.2 to 5.0 µm, and
(D) the viscosity at 20°C of the ink composition is from 0.5 to 5 mPa·s.
9. An inkjet recording method comprising spraying, by an inkjet recording system utilizing
an electrostatic field, an ink droplet of an ink composition comprising a dispersion
medium and a charged particle containing a coloring material, wherein an ink composition
having a high concentration is replenished so that the ink composition used can constantly
satisfy all of the following conditions (A) to (D):
(A) the electric conductivity at 20°C of the ink composition is from 10 to 300 nS/m,
(B) the electric conductivity at 20°C of the charged particle is 50% or more of the
electric conductivity of the ink composition,
(C) the volume average diameter of the charged particle is from 0.2 to 5.0 µm, and
(D) the viscosity at 20°C of the ink composition is from 0.5 to 5 mPa·s.
10. The inkjet recording method as claimed in the item 9, wherein the charged particle
is dispersed in the dispersion medium by using a graft polymer as a dispersant.
11. The inkjet recording method as claimed in the item 9 or 10, wherein said graft
polymer comprises a main chain moiety and a side chain moiety of a graft chain, in
which the main chain moiety is not dissolved in the dispersion medium and the side
chain moiety is dissolved in the dispersion medium.
12. The inkjet recording method as claimed in any one of the items 9 to 11, wherein
the charged particle has an electric charge amount of from 5 to 200 µC/g.
13. The inkjet recording method as claimed in any one of the items 9 to 12, which
comprises the dispersion medium in an amount of from 20 to 99 wt% based on the entire
ink composition.
14. The inkjet recording method as claimed in any one of the items 9 to 13, wherein
the coloring material is a pigment and the pigment content is from 0.1 to 50 wt% based
on the entire ink composition.
15. The inkjet recording method as claimed in any one of the items 9 to 14, wherein
the charged particle is a pigment coated with a covering agent.
Brief Description of the Drawings
[0009]
Fig. 1 is an entire constitutional view schematically showing one example of the inkjet
printing apparatus for use in the present invention.
Fig. 2 is a perspective view showing the constitution of the inkjet head of the inkjet
recording apparatus for use in the present invention (for clearly showing the constitution,
the edge of the guard electrode in each ejection part is not drawn).
Fig. 3 is a side cross-sectional view showing the distributed state of charged particles
(corresponding to the portion cut along the arrow X-X in Fig. 2) when a large number
of ejection parts are used in the inkjet head shown in Fig. 2.
Description of Symbols and Numerals
[0010]
- G
- spraying ink droplet
- P
- recording medium
- Q
- ink flow
- R
- charged particle
- 1
- inkjet recording apparatus
- 2, 2Y,
- 2M, 2C, 2K ejection head
- 3
- ink circulating system
- 4
- head driver
- 5
- position-controlling means
- 6A to 6C
- rollers for straining the conveyance belt
- 7
- conveyance belt
- 8
- conveyance belt position-detecting means
- 9
- electrostatic adsorption means
- 10
- destaticizing means
- 11
- dynamic means
- 12
- feed roller
- 13
- guide
- 14
- image fixing means
- 15
- guide
- 16
- recording medium position-detecting means
- 17
- ventilation fan ,
- 18
- solvent vapor adsorbent
- 38
- ink guide
- 40
- supporting rod part
- 42
- ink meniscus
- 44
- insulating layer
- 46
- first ejection electrode
- 48
- insulating layer
- 50
- guard electrode
- 52
- insulating layer
- 56
- second ejection electrode
- 58
- insulating layer
- 62
- floating conductive plate
- 64
- covering film
- 66
- insulating member
- 70
- inkjet head
- 72
- ink flow path
- 74
- substrate
- 75, 75A, 75B
- opening
- 76, 76A, 76B
- ejection part
- 78
- ejection part
DETAILED DESCRIPTION OF THE INVENTION
[0011] The ink composition of the present invention is used for recording an image on a
recording medium by an inkjet recording system. In the present invention, an ink composition
satisfying all of the following conditions (A) to (D) is used, whereby an ink droplet
can be always stably ejected in the inkjet recording for a long period of time:
(A) the electric conductivity at 20°C of the ink composition is from 10 to 300 nS/m,
preferably from 30 to 200 nS/m,
(B) the electric conductivity at 20°C of the charged particle is 50% or more, preferably
60% or more, of the electric conductivity of the ink composition,
(C) the volume average diameter of the charged particle is from 0.2 to 5.0 µm, preferably
from 0.3 to 3.0 µm, and
(D) the viscosity of the ink composition is from 0.5 to 5 mPa·s, preferably from 0.8
to 4 mPa·s.
[0012] The conditions to be satisfied in the ink composition of the present invention will
be described in detail, but the electric conductivity is a measured value at 20°C,
unless there is a specific definition.
[0013] In the present invention, the electric conductivity of the ink composition is from
10 to 300 nS/m. If the electric conductivity of the ink composition is less than 10
nS/m, an ink droplet cannot be ejected, whereas if it exceeds 300 nS/m, electric conduction
occurs in the head (ejection part) of an inkjet recording apparatus and the head is
damaged. The electric conductivity of the ink composition is preferably from 30 to
200 nS/m.
[0014] The electric conductivity of the charged particle is a value determined by centrifuging
the ink composition to precipitate charged particles, measuring the electric conductivity
of the supernatant and subtracting the obtained value from the electric conductivity
of the ink composition. In the present invention, the electric conductivity of the
charged particle is 50% or more of the electric conductivity of the ink composition.
In the inkjet recording system utilizing an electrostatic field, concentration of
charged particles occurs at the ejection of an ink droplet, but if the electric conductivity
of the charged particle is less than 50% of the electric conductivity of the ink composition,
insufficient concentration results and bleeding of the ink recorded on a recording
medium is caused. The electric conductivity of the charged particle is preferably
60% or more of the electric conductivity of the ink composition.
[0015] The electric charge amount of the charged particle is preferably from 5 to 200 µC/g.
If the electric charge amount is less than this range, insufficient concentration
results, whereas if it exceeds this range, excessive concentration occurs and the
ink readily clogs at the head ejection port. The electric charge amount is more preferably
from 10 to 150 µC/g, still more preferably from 15 to 100 µC/g.
[0016] The volume average diameter of charged particles can be measured, for example, by
a centrifugal precipitation method using a device such as Ultracentrifugal Automatic
Particle Analyzer CAPA-700 (manufactured by Horiba Ltd.). The average diameter includes
a volume average diameter, a number average diameter and the like according to the
calculation method, but in the present invention, the volume average diameter of charged
particles is from 0.2 to 5.0 µm. If the volume average diameter is less than 0.2 µm,
insufficient concentration of charged particles results and bleeding of the ink recorded
on a recording medium occurs, whereas if the volume average diameter exceeds 5.0 µm,
clogging is readily caused at the head ejection port. The volume average diameter
is preferably from 0.3 to 3.0 µm. The particle size distribution is preferably narrow
and uniform.
[0017] In the present invention, the viscosity of the ink composition is from 0.5 to 5 mPa·s.
If the viscosity is less than this range, the ink composition drips from the ink ejection
port of the head, whereas if it exceeds this range, an ink droplet cannot be ejected.
The viscosity is more preferably from 0.8 to 4 mPa·s. The surface tension of the ink
composition is preferably from 10 to 70 mN/m. If the surface tension is less than
this range, the ink composition drips from the ink ejection port of the head, whereas
if it exceeds this range, an ink droplet cannot be ejected. The surface tension is
more preferably from 15 to 50 mN/m.
[0018] The electric conductivity of the ink composition and the electric conductivity of
the charged particle each can be adjusted by the kind and amount added of the dispersion
medium used and the kind and amount added of the charge-control agent used. The charge-control
agent generates an electric charge by the action of ion dissociation and adsorption
or the like to the particle and therefore, greatly contributes to the electric conductivity
of the charged particle. Accordingly, the conditions (A) and (B) of the present invention
can be achieved by using a liquid having low electric conductivity or high electric
resistivity as the dispersion medium and causing a charge-control agent to impart
electric conductivity to the particle.
[0019] Some coloring materials for use in the present invention have electric conductivity,
but by coating the coloring material with a covering agent, the electric conductivity
can be masked and the control of electric conductivity to the particle by the charge-control
agent can be facilitated.
[0020] The volume average particle size of the charged particle can be adjusted by the kind
and amount added of the dispersant used and the device used for the dispersion (formation
of particles). The dispersant adsorbs to the charged particle and disperses the particles
in the dispersion medium and therefore, by appropriately adjusting the adsorption
strength of the dispersant to the charged particle and the affinity of the dispersant
for the dispersion medium, the volume average diameter can be controlled and the condition
(C) of the present invention can be achieved.
[0021] The viscosity of the ink composition can be adjusted by the kind and amount of the
dispersion medium used and the kind and amount of the polymer component dissolved
in the dispersion medium, such as dispersant. The viscosity may also be adjusted by
further using a surfactant. By appropriately using these, the condition (D) of the
present invention can be achieved.
[Dispersion Medium]
[0022] The dispersion medium is preferably a dielectric liquid having a high electric resistivity,
specifically 10
10 Ωcm or more. If a dispersion medium having a low electric resistivity is used, electric
conduction is generated between adjacent recording electrodes and this is improper.
The dielectric constant of the dielectric liquid is preferably 5 or less, more preferably
4 or less, still more preferably 3.5 or less. With a dielectric constant in such a
range, an electric field can effectively act on the charged particles in the dielectric
liquid and this is preferred.
[0023] The ink composition of the present invention comprises a dispersion medium and a
charged particle containing a coloring material. Each component of the ink composition
of the present invention will be described in detail.
[0024] The dispersion medium for use in the present invention includes a linear or branched
aliphatic hydrocarbon, an alicyclic hydrocarbon, an aromatic hydrocarbon, a halogen
substitution product of these hydrocarbons, and silicone oil. Examples thereof include
hexane, heptane, octane, isooctane, decane, isodecane, decalin, nonane, dodecane,
isododecane, cyclohexane, cyclooctane, cyclodecane, toluene, xylene and mesitylene.
Specific examples thereof include Isoper C, Isoper E, Isoper G, Isoper H, Isoper L,
Isoper M (Isoper: a trade name of Exxon Corp.), Shellsol 70, Shellsol 71 (Shellsol:
a trade name of Shell Oil Corp.), Amsco OMS solvent, Amsco 460 solvent (Amsco: a trade
name of American Mineral Spirits Co.) and KF-96L (a trade name of Shin-Etsu Silicone).
These can be used individually or in combination. The dispersion medium content is
preferably from 20 to 99 wt% based on the entire ink composition. With a dispersion
medium content of 20 wt% or more, coloring' material-containing particles can be dispersed
in the dispersion medium and with a dispersion medium content of 99 wt% or less, the
coloring material content can be satisfied.
[Coloring Material]
[0025] The coloring material which can be used in the present invention includes known dyes
and pigments. The coloring material can be selected according to use or purpose. For
example, in view of color tone of the image-recorded material (printed matter), a
pigment is preferably used (see, for example,
Ganryo Bunsan Anteika to Hyomen Shori Gijutsu·Hyoka (Stabilization of Pigment Dispersion
and Technique and Evaluation of Surface Treatment), 1st imp., Gijutsu Joho Kyokai (December 25, 2001), hereinafter sometimes referred
to as "Non-Patent Document 1"). By changing the coloring material, four color inks
of yellow, magenta, cyan and black can be prepared. In particular, when a pigment
used for the offset printing ink or proof is used, the same color tone as an offset
printed matter can be obtained and this is preferred.
[0026] Examples of the pigment for yellow ink include monoazo pigments such as C.I. Pigment
Yellow 1 and C.I. Pigment Yellow 74, disazo pigments such as C.I. Pigment Yellow 12
and C.I. Pigment Yellow 17, non-benzidine azo pigments such as C.I. Pigment Yellow
180, azo lake pigments such as C.I. Pigment Yellow 100, condensed azo pigments such
as C.I. Pigment Yellow 95, acid dye lake pigments such as C.I. Pigment Yellow 115,
basic dye lake pigments such as C.I. Pigment Yellow 18, anthraquinone-base pigments
such as Flavanthrone Yellow, isoindolinone pigments such as Isoindolinone Yellow 3RLT
and C.I. Pigment Yellow 139, quinophthalone pigments such as Quinophthalone Yellow,
isoindoline pigments such as Isoindoline Yellow, nitroso pigments such as C.I. Pigment
Yellow 153, and metal complex salt azomethine pigments such as C.I. Pigment Yellow
117.
[0027] Examples of the pigment for magenta ink include monoazo-base pigments such as C.I.
Pigment Red 3, disazo pigments such as C.I. Pigment Red 38, azo lake pigments such
as C.I. Pigment Red 53:1 and C.I. Pigment Red 57:1, condensed azo pigments such as
C.I. Pigment Red 144, acid dye lake pigments such as C.I. Pigment Red 174, basic dye
lake pigments such as Pigment Red 81, anthraquinone-base pigments such as C.I. Pigment
Red 177, thioindigo pigments such as C.I. Pigment Red 88, perynone pigments such as
C.I. Pigment Red 194, perylene pigments such as C.I. Pigment Red 149, quinacridone
pigments such as C.I. Pigment Red 122, isoindolinone pigments such as C.I. Pigment
Red 180, and alizarin lake pigments such as C.I. Pigment Red 83.
[0028] Examples of the pigment for cyan ink include disazo-base pigments such as C.I. Pigment
Blue 25, phthalocyanine pigments such as C.I. Pigment Blue 15, acid dye lake pigments
such as C.I. Pigment Blue 24, basic dye lake pigments such as Pigment Blue 1, anthraquinone-base
pigments such as C.I. Pigment Blue 60, and alkali blue pigments such as C.I. Pigment
Blue 18.
[0029] Examples of the pigment for black ink include organic pigments such as aniline black-base
pigment, iron oxide pigments, and carbon black pigments such as furnace black, lamp
black, acetylene black and channel black.
[0030] Furthermore, processed pigments represented by microlith pigments such as microlith-A,
microlith-K and microlith-T can also be suitably used. Specific examples thereof include
Micolith Yellow 4G-A, Microlith Red BP-K, Microlith Blue 4G-T and Microlith Black
C-T.
[0031] In addition, various pigments can be used, if desired, for example, calcium carbonate
and titanium oxide may be used as the pigment for white ink, aluminum powder may be
used as the pigment for silver ink, and copper alloy may be used as the pigment for
gold ink.
[0032] In view of easiness in the production of ink, it is fundamentally preferred to use
one pigment for one color. However, depending on the case, two or more pigments are
preferably used in combination for adjusting the color hue, for example, phthalocyanine
is mixed with carbon black for the preparation of black ink. Also, the pigment may
be used after surface-treating it by a known method such as rosin treatment (see,
Non-Patent Document 1,
supra).
[0033] The pigment content is preferably from 0.1 to 50 wt% based on the entire ink composition.
With a pigment content of 0.1 wt% or more, the amount of pigment is satisfied and
satisfactory color formation can be obtained in the printed matter and with a pigment
content of 50 wt% or less, coloring material-containing particles can be successfully
dispersed in the dispersion medium. The pigment content is more preferably from 1
to 30 wt%.
[Covering Agent]
[0034] In the present invention, the coloring material such as pigment is preferably dispersed
(formed into particles) in a state of being covered with a covering agent than the
direct dispersion (formation of particles) in the dispersion medium. By covering the
coloring material with a covering agent, the electric charge of the coloring material
is masked and preferred charging property can be imparted. Furthermore, the difference
in the dispersion stability which varies according to the kind of the coloring material
can be eliminated and the same dispersion stability can be imparted. Furthermore,
in the present invention, the ink after inkjet recording on a recording medium is
preferably fixed by a heating device such as heat roller and at this time, the covering
agent is melted due to heat, thereby realizing efficient fixing.
[0035] Examples of the covering agent include rosins, phenol resin, rosin-modified phenol
resin, alkyd resin, (meth)acryl-base polymer, polyurethane, polyester, polyether,
polyamide, polyethylene, polybutadiene, polystyrene, polyvinyl acetate, acetal-modified
product of polyvinyl alcohol, and polycarbonate. Among these, in view of easiness
of particle formation, polymers having a weight average molecular weight of 2,000
to 1,000,000 and a polydispersion degree (weight average molecular weight/number average
molecular weight) of 1.0 to 5.0 are preferred. Furthermore, in view of easiness of
fixing, polymers where any one of the softening point, the glass transition point
and the melting point is from 40 to 120°C are preferred.
[0036] The polymer which is particularly suitably used as the covering agent in the present
invention is a polymer containing any one of the constituent units represented by
the following formula (1) to (4).

wherein X
11 represents an oxygen atom or -N (R
13) -, R
11 represents a hydrogen atom or a methyl group, R
12 represents a hydrocarbon group having from 1 to 30 carbon atoms, R
13 represents a hydrogen atom or a hydrocarbon group having from 1 to 30 carbon atoms,
R
21 represents a hydrogen atom or a hydrocarbon group having from 1 to 20 carbon atoms,
and R
31, R
32 and R
41 each independently represents a divalent hydrocarbon group having from 1 to 20 carbon
atoms, provided that the hydrocarbon groups of R
12, R
21, R
31, R
32 and R
41 each may contain an ether bond, an amino group, a hydroxy group or a halogen-substituted
group.
[0037] The polymer containing the constituent unit represented by formula (1) can be obtained
by radical-polymerizing a corresponding radical polymerizable monomer according to
a known method. Examples of the radical polymerizable monomer used include (meth)acrylic
acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate,
butyl (meth)acrylate, hexyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate,
dodecyl (meth)acrylate, stearyl (meth)acrylate, cyclohexyl (meth)acrylate, phenyl
(meth) acrylate, benzyl (meth)-acrylate and 2-hydroxyethyl (meth)acrylate, and (meth)acrylamides
such as N-methyl(meth)acrylamide, N-propyl(meth)acrylamide, N-phenyl(meth)acrylamide
and N,N-dimethyl(meth)acrylamide.
[0038] The polymer containing the constituent unit represented by formula (2) can be obtained
by radical-polymerizing a corresponding radical polymerizable monomer according to
a known method. Examples of the radical polymerizable monomer used include ethylene,
propylene, butadiene, styrene and 4-methylstyrene.
[0039] The polymer containing the constituent unit represented by formula (3) can be obtained
by dehydration-condensing a corresponding dicarboxylic acid or acid anhydride and
a diol according to a known method. Examples of the dicarboxylic acid used include
succinic anhydride, adipic acid, sebacic acid, isophthalic acid, terephthalic acid,
1,4-phenylenediacetic acid and diglycol. Examples of the diol used include ethylene
glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,10-decanediol,
2-butene-1,4-diol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, 1,4-benzenedimethanol
and diethylene glycol.
[0040] The polymer containing the constituent unit represented by formula (4) can be obtained
by dehydration-condensing a carboxylic acid having a corresponding hydroxy group according
to a known method or by ring-opening polymerizing a cyclic ester of a carboxylic acid
having a corresponding hydroxy group according to a known method. Examples of the
carboxylic acid having a hydroxyl group or the cyclic ester thereof used include 6-hydroxyhexanoic
acid, 11-hydroxyundecanoic acid, hydroxybenzoic acid and ε-caprolactone.
[0041] The polymer containing at least any one of the constituent units represented by formulae
(1) to (4) may be a homopolymer of the constituent unit represented by formula (1),
(2), (3) or (4) or may be a copolymer with another constituent component. As the covering
agent, these polymers may used individually or in combination of two or more thereof.
[0042] The covering agent content is preferably from 0.1 to 40 wt% based on the entire ink
composition. With a covering agent content of 0.1 wt% or more, the amount of the covering
agent is satisfied and sufficiently high fixing property can be obtained and with
a covering agent content of 40 wt% or less, a particle containing a coloring material
and a covering agent can be successfully formed.
[Dispersant]
[0043] In the present invention, a mixture of the coloring material and the covering agent
is preferably dispersed (formed into particles) in a dispersion medium and for controlling
the particle diameter and preventing the precipitation of particles, a dispersant
is more preferably used.
[0044] Suitable examples of the dispersant include surfactants represented by sorbitan fatty
acid esters such as sorbitan monooleate, and polyethylene glycol fatty acid esters
such as polyoxyethylene distearate. Other examples include a copolymer of styrene
and maleic acid, an amine-modified product thereof, a copolymer of styrene and (meth)acryl
compound, a (meth)acryl-base polymer, a copolymer of polyethylene and (meth)acryl
compound, rosin, BYK-160, 162, 164, 182 (polyurethane-base polymers produced by Byk-Chemie),
EFKA-401, 402 (acryl-base polymers produced by EFKA), and Solsperse 17000, 24000 (polyester-base
polymers produced by Zeneca). In the present invention, in view of long-term storage
stability of the ink composition, a polymer having a weight average molecular weight
of 1,000 to 1,000,000 and a polydispersion degree (weight average molecular weight/number
average molecular weight) of 1.0 to 7.0 is preferred, and a graft polymer or a block
polymer is most preferred.
[0045] The polymer which is particularly preferably used in the present invention is a graft
polymer comprising at least a polymer component containing at least either one of
the constituent units represented by the following formulae (5) and (6), and a polymer
component containing a constituent unit represented by the following formula (7) at
least as a graft chain.

wherein X
51 represents an oxygen atom or -N (R
53) -, R
51 represents a hydrogen atom or a methyl group, R
52 represents a hydrocarbon group having from 1 to 10 carbon atoms, R
53 represents a hydrogen atom or a hydrocarbon group having from 1 to 10 carbon atoms,
R
61 represents a hydrogen atom, a hydrocarbon group having from 1 to 20 carbon atoms,
a halogen atom, a hydroxyl group or an alkoxy group having from 1 to 20 carbon atoms,
X
71 represents an oxygen atom or -N(R
73)-, R
71 represents a hydrogen atom or a methyl group, R
72 represents a hydrocarbon group having from 4 to 30 carbon atoms, and R
73 represents a hydrogen atom or a hydrocarbon group having from 1 to 30 carbon atoms,
provided that the hydrocarbon groups of R
52 and R
72 each may contain an ether bond, an amino group, a hydroxy group or a halogen-substituted
group.
[0046] The above-described graft polymer can be obtained by polymerizing a radical polymerizable
monomer corresponding to formula (7) preferably in the presence of a chain transfer
agent, introducing a polymerizable functional group into the terminal of the polymer
obtained, and copolymerizing the polymer with a radical polymerizable monomer corresponding
to formula (5) or (6).
[0047] Examples of the radical polymerizable monomer corresponding to formula (5) include
(meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl
(meth)acrylate, butyl (meth)acrylate, hexyl (meth)-acrylate, cyclohexyl (meth)acrylate,
phenyl (meth)acrylate, benzyl (meth)acrylate and 2-hydroxyethyl (meth)acrylate, and
(meth)acrylamides such as N-methyl(meth)acrylamide, N-propyl(meth)acrylamide, N-phenyl(meth)acrylamide
and N,N-dimethyl(meth)acrylamide.
[0048] Examples of the radical polymerizable monomer corresponding to formula (6) include
styrene, 4-methylstyrene, chlorostyrene and methoxystyrene. Also, examples of the
radical monomer corresponding to formula (6) include hexyl (meth)acrylate, octyl (meth)acrylate,
2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, stearyl (meth)-acrylate.
[0049] The graft polymer comprising a polymer component containing at least either one of
the constituent units represented by formulae (5) and (6) and a polymer component
containing a constituent unit represented by formula (7) at least as a graft chain
may have only the constituent units represented by formula (5) and/or (6) and formula
(7) or may contain another constituent component. The compositional ratio of the polymer
component having a graft chain and other polymer component is preferably from 10:90
to 90:10. Within this range, good particle-forming property is obtained and a desired
particle diameter is advantageously easy to obtain. As the dispersant, these polymers
may be used individually or in combination of two or more thereof.
[0050] In the present invention, the following graft polymer (may be referred as "graft
polymer A" hereinafter) is still more preferably used as a dispersant, in view of
stably and constantly ejecting a ink droplet in a long-period ink jet recording. The
graft polymer A for use in the present invention is preferably a polymer having a
weight average molecular weight of 1,000 or more and containing, as a side chain,
a graft chain which is a polymer component having a weight average molecular weight
of 500 or more. The graft polymer A particularly preferred as the dispersant is a
polymer where the main chain moiety is not dissolved in the dispersion medium and
the side chain moiety (graft chain) is dissolved in the dispersion medium.
[0051] The term "the main chain moiety is not dissolved in the dispersion medium" means
that a polymer constituted only by a main chain part and having no side chain part
is not dissolved in the dispersion medium. Specifically, this polymer preferably has
a solubility of 3 g or less in 100 g of the dispersion medium.
[0052] The term "the side chain moiety is dissolved in the dispersion medium" means that
a polymer constituted only by a side chain part and having no main chain part is dissolved
in the dispersion medium. Specifically, this polymer preferably has a solubility of
5 g or more in 100 g of the dispersion medium.
[0053] The graft polymer where the main chain part is not dissolved and the side chain part
is dissolved, is dissolved or dispersed in the dispersion medium while changing the
transparent state into a white turbid state. When such a graft polymer is used, the
main chain part is not dissolved in the dispersion medium and strongly adsorbs to
the charged particle, whereas the side chain part is dissolved in the dispersion medium
and thereby increased in the affinity for the dispersion medium, as a result, the
dispersibility of charged particles in the dispersion medium is enhanced.
[0054] The graft polymer A which can be suitably used in the present invention is a polymer
having a weight average molecular weight of 1,000 or more and containing at least
a constituent unit represented by the following formula (8) and a constituent unit
represented by the following formula (9) :

wherein
R
151, R
152, R
161 and R
162, which may be the same or different, each represents a hydrogen atom or a methyl
group,
R
153 represents a hydrogen atom or a hydrocarbon group having from 1 to 30 carbon atoms,
which may have a substituent, provided that the hydrocarbon group represented by R
153 may contain an ether bond, an ester bond, an amide bond, a carbamate bond, a hydroxyl
group, or a halogen-substituted group,
X
151 and X
161, which may be the same or different, each represents a single bond or a divalent
linking group having a total atom number of 50 or less and comprising two or more
atoms selected from C, H, N, O, S and P, and
G represents a polymer component having a weight average molecular weight of 500
or more and containing at least the constituent unit represented by the following
formula (10), or a polydimethylsiloxane group having a weight average molecular weight
of 500 or more:

wherein
R
171 and R
172, which may be the same or different, each represents a hydrogen atom or a methyl
group,
R
173 represents a hydrogen atom or a hydrocarbon group having from 1 to 30 carbon atoms,
provided that the hydrocarbon group represented by R
173 may contain an ether bond, an ester bond, an amide bond, a carbamate bond, an amino
group, a hydroxyl group or a halogen-substituted group, and
X
171 represents a single bond or a divalent linking group having a total atom number of
50 or less and comprising two or more atoms selected from C, H, N, O, S and P.
[0055] In view of stability of ink ejection, the total atom number of R
173 is preferably larger than the total atom number of R
153.
[0056] The graft polymer containing at least a constituent unit represented by formula (8)
and a constituent unit represented by formula (9), which is suitably used in the present
invention, can be obtained by polymerizing a radical polymerizable monomer corresponding
to formula (8) and a radical polymerizable macromonomer corresponding to formula (9)
by using a known radical polymerization initiator. The monomer corresponding to formula
(8) is a monomer represented by the following formula (8M) and the macromonomer corresponding
to formula (9) is a macromonomer represented by the following formula (9M):

wherein each symbol has the same meaning as in formulae (8) and (9).
[0057] Examples of the monomer represented by formula (8M) include (meth)acrylic acid esters
such as methyl (meth)-acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl
(meth)acrylate, hexyl (meth)acrylate, octyl (meth)-acrylate, 2-ethylhexyl (meth) acrylate,
cyclohexyl (meth)-acrylate, dodecyl (meth)acrylate, stearyl (meth) acrylate, phenyl
(meth)acrylate, benzyl (meth)acrylate, 2-methoxyethyl (meth)acrylate and 2-hydroxyethyl
(meth)acrylate, (meth)acrylamides such as N-methyl(meth)acrylamide, N-propyl(meth)acrylamide,
N-phenyl(meth)acrylamide and N,N-dimethyl(meth)acrylamide, styrenes such as styrene,
methylstyrene, chlorostyrene and methoxystyrene, hydrocarbons such as 1-butene, vinyl
acetates, vinyl ethers and vinylpyridines.
[0058] The macromonomer represented by formula (9M) is a polymer having a radical polymerizable
functional group at the terminal, which is obtained by polymerizing a radical polymerizable
monomer corresponding to formula (10) and represented by the following formula (10M)
, if desired, in the presence of a chain transfer agent and then introducing a radical
polymerizable functional group into the terminal of the polymer obtained.
[0059] The macromonomer represented by formula (9M) is preferably a macromonomer having
a weight average molecular weight of 500 to 500,000 and a polydispersion degree (weight
average molecular weight/number average molecular weight) of 1.0 to 7.0. The macromonomer
represented by formula (9M) may be a polydimethylsiloxane having a radical polymerizable
functional group at the terminal.

wherein each symbol has the same meaning as in formula (10).
[0060] Examples of the monomer represented by formula (10M) include (meth)acrylic acid esters
such as methyl (meth)-acrylate, ethyl (meth) acrylate, propyl (meth)acrylate, butyl
(meth) acrylate, hexyl (meth) acrylate, octyl (meth)-acrylate, 2-ethylhexyl (meth)
acrylate, cyclohexyl (meth)-acrylate, dodecyl (meth) acrylate, stearyl (meth) acrylate,
phenyl (meth)acrylate, benzyl (meth)acrylate and 2-hydroxyethyl (meth)acrylate, (meth)acrylamides
such as N-methyl(meth)acrylamide, N-propyl(meth)acrylamide, N-phenyl(meth)acrylamide
and N,N-dimethyl(meth)acrylamide, styrenes such as styrene, methylstyrene, chlorostyrene
and methoxystyrene, hydrocarbons such as 1-butene, vinyl acetates, vinyl ethers and
vinylpyridines.
[0061] The graft polymer A for use in the present invention may have only the constituent
units represented by formulae (8) and (9) or may contain another constituent component.
Specific examples of the graft polymer A, which is suitably used in the present invention
include Polymers [BZ-1] to [BZ-8] represented by the following structural formulae.

(in formulae, n shows that the compound is a polymer, and for example, is a number
of 5 or more).
[0062] The term "the main chain moiety is not dissolved in the dispersion medium" means,
for example, that the polymer not containing the constituent unit represented by formula
(9) has a solubility of 3 g or less in 100 g of the dispersion medium, and the term
"the side chain moiety is dissolved in the dispersion medium" means that the polymer
represented by G in formula (9) or the macromonomer represented by formula (9M) has
a solubility of 5 g or more in 100 g of the dispersion medium.
[0063] In the present invention, in view of long-term storage stability and ink ejection
stability of the ink composition, the dispersant is preferably a graft polymer A having
a weight average molecular weight of 1,000 to 1,000,000 and a polydispersion degree
(weight average molecular weight/number average molecular weight) of 1.0 to 7.0.
[0064] The weight average molecular weight of the graft polymer A is preferably 1.5 times
or more the weight average molecular weight of the graft chain (preferably the macromer
component represented by formula (10)).
[0065] The mass ratio of the unit constituting the main chain and the unit constituting
the graft chain is preferably from 30:70 to 95:5. As the dispersant, these polymers
may be used individually or in combination of two or more thereof.
[0066] The dispersant content is preferably from 0.01 to 30 wt% based on the entire ink
composition. Within this range, good particle-forming property and a desired particle
diameter can be obtained. In addition, within this range, ink particles are not aggregated
in the ink ejection head or circulating pump, thereby preventing the head or pump
from clogging, and a desired particle diameter can be obtained to facilitate the ejection
at the inkjet recording.
[Charge-Control Agent]
[0067] In the present invention, a mixture of a coloring material and a covering agent is
dispersed (formed into particles) in a dispersion medium preferably by using a dispersant
and for controlling the electric charge amount of the particle, more preferably by
using a charge-control agent in combination.
[0068] Suitable examples of the dispersant include metal salts of organic carboxylic acid,
such as zirconium naphthenate and zirconium octenate, ammonium salts of organic carboxylic
acid, such as tetramethylammonium stearate, metal salts of organic sulfonic acid,
such as sodium dodecylbenzenesulfonate and magnesium dioctylsulfosuccinate, ammonium
salts of organic sulfonic acid, such as tetrabutylammonium toluenesulfonate, polymers
having a carboxylic acid group in the side chain, such as carboxylic acid group-containing
polymer obtained by modifying a copolymer of styrene and maleic anhydride with amine,
polymers having a carboxylic acid anion group in the side chain, such as copolymer
of stearyl methacrylate and tetramethylammonium salt of methacrylic acid, polymers
having a nitrogen atom in the side chain, such as copolymer of styrene and vinylpyridine,
and polymers having an ammonium group in the side chain, such as copolymer of butyl
methacrylate and N-(2-methacryloyloxyethyl)-N,N,N-trimethylammonium tosylate. The
electric charge imparted to the particle may be a positive electric charge or a negative
electric charge. The dispersant content is preferably from 0.0001 to 10 wt% based
on the entire ink composition. Within this range, the electric conductivity of the
ink composition can be easily adjusted to the range from 10 to 300 nS/m and also,
the electric conductivity of the charged particle can be easily adjusted to 50% or
more of the electric conductivity of the ink composition.
[Other Components]
[0069] In the present invention, for example, an antiseptic for preventing putrefaction
and a surfactant for controlling the surface tension can be further contained according
to the purpose.
[Preparation of Charged Particle]
[0070] The ink composition containing a charged particle of the present invention can be
prepared by using these components and dispersing (particle-forming) the coloring
material and preferably the covering agent. Examples of the dispersing (particle-forming)
method include the followings:
(1) the coloring material and the covering agent are previously mixed and then dispersed
(formed into particles) by using the dispersant and the dispersion medium and thereto,
the charge-control agent is added,
(2) the coloring material, the covering agent, the dispersant and the dispersion medium
are simultaneously used and dispersed (formed into particles) and thereto, the charge-control
agent is added, and
(3) the coloring material, the covering agent, the dispersant, the charge-control
agent and the dispersion medium are simultaneously used and dispersed (formed into
particles).
[0071] Examples of the device used for the mixing or dispersion include kneader, dissolver,
mixer, high-speed disperser, sand mill, roll mill, ball mill, attritor and bead mill
(see, Non-Patent Document 1,
supra).
[Inkjet Recording Apparatus]
[0072] In the present invention, the above-described ink composition is used for recording
an image on a recording medium by an inkjet recording system. In the present invention,
an inkjet recording system utilizing an electrostatic field is preferably employed.
The inkjet recording system utilizing an electrostatic field is a system where a voltage
is applied between a control electrode and a back electrode on the back surface of
a recording medium, as a result, the charged particles in the ink composition are
concentrated at the ejection site by the electrostatic force and flown to a recording
medium from the ejection site. In applying a voltage between a control electrode and
a back electrode, for example, when the charged particle is positive, the control
electrode serves as the positive electrode and the back electrode serves as the negative
electrode. The same effect can be obtained also by electrifying the recording medium
instead of applying a voltage to the back electrode.
[0073] The ink spraying system include, for example, a system of spraying an ink from a
needle-like tip such as injection needle and by using the ink composition of the present
invention, recording can be performed. However, after charged particles are concentrated
and ejected, the charged particle is difficult to supply and stable recording can
be hardly performed for a long period of time. If the ink is circulated to enforcedly
supply the charged particle, the ink overflows from the injection needle-like tip
and the meniscus shape at the injection needle-like tip as the ejection site is not
stabilized, as a result, the recording cannot be stably performed. Therefore, this
system is suitable for short-term recording.
[0074] On the other hand, a method of circulating an ink composition without causing the
ink composition to overflow from the ejection opening is preferably used. For example,
in a method where an ink is circulated in an ink chamber having an ejection opening
and when a voltage is applied to a control electrode formed in the periphery of the
ejection opening, a concentrated ink droplet flies from the tip of an ink guide present
in the ejection opening and having a tip facing the recording medium side, supply
of charged particles by the circulation of ink and meniscus stability of the ejection
site both can be established at the same time and therefore, the recording can be
stably performed for a long period of time. Furthermore, in this system, the portion
of the ink coming into contact with an outside air is only the ejection opening and
is very small, so that the solvent can be prevented from evaporation and the physical
properties of ink can be stabilized. Therefore, this system can be suitably used in
the present invention.
[0075] A constitution example of the inkjet recording apparatus to which the ink composition
of the present invention is suitably applied is described below.
[0076] An apparatus of performing one-side four-color printing on a recording medium, shown
in Fig. 1, is briefly described below.
[0077] The inkjet recording apparatus 1 shown in Fig. 1 comprises an ink circulating system
3 which supplies an ink to an ejection head 2 constituted by ejection heads 2C, 2M,
2Y and 2K of four colors for performing the formation of a full color image and further
recovers the ink from the ejection head 2, a head driver 4 for driving the ejection
head 2 by the output from an external device (not shown) such as computer and RIP,
and position controlling means 5. Furthermore, the inkjet recording apparatus 1 comprises
a conveyance belt 7 strained by three rollers 6A, 6B and 6C, conveyance belt position-detecting
means 8 constituted by an optical sensor or the like capable of detecting the position
in the cross direction of the conveyance belt 7, electrostatic adsorption means 9
for holding a recording medium P on the conveyance belt, and destaticizing means 10
and dynamic means 11 for separating the recording medium P from the conveyance 7 after
the completion of image formation. Upstream and downstream the conveyance belt 7,
a feed roller 12 and a guide 13 for feeding the recording medium P to the conveyance
belt 7 from a stocker (not shown), and fixing means 14 and a guide 15 for fixing the
ink on the recording medium P separated and at the same time, conveying the recording
medium to a discharged paper stocker are disposed. Also, in the inside of the inkjet
printing apparatus 1, recording medium position-detecting means 16 is disposed at
the position opposing the ejection head 2 through the conveyance belt 7, a solvent
recovery part for recovering solvent vapor generated from the ink composition, comprising
a ventilating fan 17 and a solvent vapor adsorbent 18, is disposed and the vapor inside
the apparatus is discharged to the outside of the apparatus through the recovery part.
[0078] The feed roller 12 can be a known roller and is disposed to enhance the feeding ability
for the recording medium. On the recording medium P, dirt, paper dust and the like
are sometimes attached and these are preferably removed. The recording medium P fed
by the feed roller is conveyed to the conveyance belt 7 through the guide 13. The
back surface (preferably metal back surface) of the conveyance belt 7 is placed through
a roller 6A. The recording medium conveyed is electrostatically adsorbed on the conveyance
belt by the electrostatic adsorption means 9. In Fig. 1, the electrostatic adsorption
is performed by a scorotron charger connected to a negative high voltage power source.
By the electrostatic adsorption means 9, the recording medium 9 is electrostatically
adsorbed on the conveyance belt 7 without floating and at the same time, the recording
medium surface is uniformly electrified. In this example, the electrostatic adsorption
means is used also as the electrification means for the recording medium, but the
electrification means may be provided separately. The electrified recording medium
P is conveyed to the ejection head part by the conveyance belt 7 and recording signal
voltages are superposed while using the electrification potential as a bias, whereby
an electrostatic inkjet image is formed. The recording medium P having formed thereon
an image is destaticized by the destaticizing means 10, separated from the conveyance
belt 7 by the dynamic means 11 and conveyed to the fixing part. The recording medium
P separated is transferred to the image fixing means 14 and the image is fixed. The
recording medium P after fixing is discharged to a discharged paper stocker (not shown)
through the guide 15. This apparatus also has means for recovering solvent vapor generated
from the ink composition. The recovery means comprises a solvent vapor absorbent 18
and the gas containing solvent vapor in the apparatus is introduced into the adsorbent
by the 'ventilating fan 17 and after adsorbing and recovering the vapor, discharged
outside the apparatus. The inkjet recording apparatus is not limited to this example,
but the number, shape, relative disposition, charged polarity and the like of constituent
devices such as roller and charger can be arbitrarily selected. Furthermore, the system
is described here by referring to the drawing of a four-color image, but a system
of drawing an image having more colors by using a light color ink or a special color
ink in combination may be employed.
[0079] The inkjet recording apparatus for use in the above-described inkjet printing method
comprises an ejection head 2 and an ink circulating system 3 and furthermore, the
ink circulating system 3 has an ink tank, an ink circulating device, an ink concentration-controlling
device, an ink temperature-controlling device and the like. In the ink tank, a stirring
device may be contained.
[0080] The ejection head 2 may be a single channel head, a multi-channel head or a full
line head and the main scanning is performed by the rotation of a conveyance belt
7.
[0081] The inkjet head which is suitably used in the present invention performs the inkjet
recording by electrophoresing charged particles in the ink flow path to increase the
ink concentration in the vicinity of the opening and thereby ejecting an ink droplet,
where the ink droplet is ejected by an electrostatic suction force mainly ascribable
to the recording medium or the opposing electrode disposed on the back surface of
the recording medium. Therefore, in the case where the recording medium or opposing
electrode is not facing the head or even if facing the head, a voltage is not applied
to the recording medium or opposing electrode, an ink droplet is not ejected even
when a voltage is applied by mistake to the ejection electrode or vibration is applied,
and the inside of the apparatus is not stained.
[0082] Figs. 2 and 3 show an ejection head which is suitably used for the above-described
inkjet apparatus. As shown in Figs. 2 and 3, the inkjet head 70 comprises an electrically
insulating substrate 74 constituting the top wall of an ink flow path 72 where a one-way
ink flow Q is formed, and a plurality of ejection parts 76 of ejecting an ink toward
a recording medium P. In all of the ejection parts 76, an ink guide part 78 of guiding
an ink droplet G spraying from the ink flow path 72 toward the recording medium P
is provided. In the substrate 74, openings 75 each for allowing the ink guide part
78 to penetrate are formed and an ink meniscus 42 is formed between the ink guide
part 78 and the inner wall surface of the opening 75. The gap d between the ink guide
part 78 and the recording medium P is preferably on the order of 200 to 1,000 µm.
The ink guide part 78 is fixed in its lower end side to a supporting rod part 40.
[0083] The substrate 74 has an insulating layer 44 of separating two ejection electrodes
at a predetermined distance and thereby establishing electrical insulation, a first
ejection electrode 46 formed on the top side of the insulating layer 44, an insulating
layer 48 covering the first ejection electrode 46, a guard electrode 50 formed on
the top side of the insulating layer 48, and an insulating layer 52 covering the guard
electrode 50. Furthermore, the substrate 74 has a second ejection electrode 56 formed
on the bottom side of the insulating layer 44 and an insulating layer 58 covering
the second ejection electrode 56. The guard electrode 50 is provided for preventing
adjacent ejection parts from being affected in view of electric field by the voltage
applied to the first ejection electrode 46 or the second ejection electrode 56.
[0084] Furthermore, in the inkjet head 70, a floating conductive plate 62 is provided in
the electrically floating state, which constitutes the bottom surface of the ink flow
path 72 and at the same time, drifts positively charged ink particles (charged particles)
R in the ink flow path 72 toward the upper side (namely, toward the recording medium
side) by using an induced voltage constantly generated due to the pulsed ejection
voltage applied to the first ejection electrode 46 and the second ejection electrode
56. On the surface of the floating conductive plate 62, an electrically insulating
covering film 64 is formed to prevent the physical properties or components of the
ink from becoming unstable as a result of, for example, injection of electric charge
into the ink. The electric resistance of the insulating covering film is preferably
10
12 Ω·cm or more, more preferably 10
13 Ω·cm or more. Also, the insulating covering film is preferably corrosion-resistant
against ink and by virtue of this resistance, the floating conductive plate 62 can
be prevented from corrosion by the ink. Furthermore, the bottom side of the floating
conductive plate 62 is covered by an insulating member 66 and by virtue of such a
constitution, the floating conductive plate 62 is completely in an electrically insulated
state.
[0085] One or more floating conductive plate 62 is provided per one head unit (for example,
when C, M, Y and K four heads are present, each head has at least one floating conductive
plate and a floating conductive plate is not commonly used between C and M head units)
.
[0086] For spraying an ink from the inkjet head 70 as shown in Fig. 3 and recording an image
on the recording medium P, an ink flow Q is generated by circulating the ink in the
ink flow path 72 and in this state, a predetermined voltage (for example, +100 V)
is applied to the guard electrode 50. Also, a positive voltage is applied to the first
ejection electrode 46, the second ejection electrode 56 and the recording medium P
such that a spraying electric field high enough to cause positively charged particles
R in the ink droplet G guided by the ink guide part 78 and flown from the opening
75 to gravitate to the recording medium P is formed between the first ejection electrode
46 and the recording medium P and between the second ejection electrode 56 and the
recording medium P (as a standard, to form a potential difference of approximately
from 1 to 3.0 kV when the gap d is 500 µm).
[0087] In this state, when a pulse voltage is applied to the first ejection electrode 46
and the second ejection electrode 56 according to image signals, the ink droplet G
elevated in the charged particle concentration is ejected from the opening 75 (for
example, when the initial concentration of charged particles is from 3 to 15%, the
charged particle concentration in the ink droplet G becomes 30% or more) .
[0088] At this time, the voltage value applied to the first ejection electrode 46 and the
second ejection electrode 56 is adjusted such that the ink droplet G is ejected only
when a pulse voltage is applied to both the first ejection electrode 46 and the second
ejection electrode 56.
[0089] When a pulsed positive voltage is applied, the ink droplet G guided by the ink guide
part 78 is flown from the opening 75 and attached to the recording medium P and at
the same time, a positive induced voltage is generated in the floating conductive
plate 62 due to the positive voltage applied to the first ejection electrode 46 and
the second ejection electrode 56. Even when the voltage applied to the first ejection
electrode 46 and the second ejection electrode 56 is pulsed, the induced voltage is
nearly a stationary voltage. Accordingly, the charged particles R positively charged
in the ink flow path 72 are moved 'upward by the force of the electric field formed
between the floating conductive plate 62 and the recording medium P and between the
guard electrode 50 and the recording medium P, and the concentration of charged particles
R increases in the vicinity of the substrate 74. As shown in Fig. 3, when a large
number of ejection parts (that is, channels for ejecting an ink droplet) are used,
the number of charged particles necessary for the ejection becomes large, but since
the first ejection electrode 46 and the second ejection electrode 56 each is increased
in the number of sheets used, the induced voltage generated in the floating conductive
plate 62 is elevated and the number of charged particles R moving to the recording
medium side increases.
[0090] In the example described above, the color particle is positively charged, but the
color particle may be negatively charged and in this case, the charged polarities
all are reversed.
[0091] In the present invention, the ink ejected on a recording medium is preferably fixed
by appropriate heating means. Examples of the heating means which can be used include
contact-type heating devices such as heat roller, heat block and belt heating, and
non-contact type heating devices such as drier, infrared lamp, visible light lamp,
ultraviolet lamp and hot air-type oven. Such a heating device is preferably continued
to and integrated with the inkjet recording apparatus. The temperature of the recording
medium at the fixing is preferably from 40 to 200°C in view of easiness of fixing.
The fixing time is preferably from 1 micro-second to 20 seconds.
[Replenishment of Ink Composition]
[0092] In the inkjet recording system utilizing an electrostatic field, the charged particles
in the ink composition are concentrated and ejected. Accordingly, when the ink composition
is ejected for a long period of time, the amount of charged particles in the ink composition
decreases and the electric conductivity of the ink composition decreases. Also, the
ratio between the electric conductivity of the charged particle and the electric conductivity
of the ink composition changes. Furthermore, at the ejection, a charged particle having
a large diameter tends to be more preferentially ejected than a charged particle having
a small diameter and therefore, the average diameter of charged particles decreases.
In addition, the content of solid matters in the ink composition changes and therefore,
the viscosity also changes.
[0093] As a result of these changes in the physical values, ejection failure may occur,
the optical density of recorded image may decrease or the bleeding of ink may be generated.
By replenishing an ink composition having a higher concentration (the concentration
of solid contents is higher) than the ink composition initially charged into the ink
tank, the amount of charged particles can be prevented from decreasing and the electric
conductivity of the ink composition and the ratio of the electric conductivity of
the charged particle to the electric conductivity of the ink composition can be kept
constant. Also, the average particle diameter and the viscosity can be maintained.
Furthermore, by keeping constant the physical values of the ink composition, the ink
is stably and uniformly ejected for a long period of time. The replenishment at this
time is preferably performed mechanically or manually, for example, by detecting the
physical values of the ink solution on use, such as electric conductivity and optical
density, and calculating the shortfall. Also, the replenishment may be performed mechanically
or manually by calculating the amount of the ink composition used, based on the image
data.
[Recording Medium]
[0094] In the present invention, various recording mediums can be used according to uses.
For example, when paper, plastic film, metal, paper laminated or vapor-deposited with
plastic or metal, plastic film laminated or vapor-deposited with metal, or the like
is used, a printed matter can be directly obtained by the inkjet recording. Also,
for example, when a support obtained by roughening the surface of a metal such as
aluminum is used, an offset printing plate can be obtained. Furthermore, when a plastic
support or the like is used, a color filter for flexographic printing plate or liquid
crystal screen can be obtained. The recording medium may be have a planar shape such
as sheet form, or may have a steric shape such as cylindrical form. When a silicon
wafer or a circuit board is used as the recording medium, this can be applied to the
production of a semiconductor or a printed circuit board.
[Fixing]
[0095] In the present invention, the ink ejected on a recording medium is preferably fixed
by appropriate heating means. Examples of the heating means which can be used include
contact-type heating devices such as heat roller, heat block and belt heating, and
non-contact type heating devices such as drier, infrared lamp, visible light lamp,
ultraviolet lamp and hot air-type oven. Such a heating device is preferably connected
and integrated with the inkjet recording apparatus.
[0096] By using the above-described ink composition of the present invention, an ink droplet
can be stably ejected for a long period of time. Furthermore, by using the above-described
inkjet recording apparatus, an image-recorded material free from bleeding of ink and
having high image density and high image quality can be obtained over a long period
of time. The factor contributing to these effects is not clearly known, but for example,
these effects are presumed to result because the particles obtained by using a graft
polymer A as the dispersant are stably present without undergoing aggregation due
to shearing stress generated on the inner wall of head at the ejection of ink or concentration.
Also, this factor is presumed to be attributable to the fact that the graft polymer
A of the present invention has high adsorption strength to the covering agent or coloring
material and at the same time, has high dissolving ability in a dispersion medium.
Furthermore, the image-recorded materials (printed matters) after inkjet recording
and fixing can be prevented from blocking even when these are superposed one on another,
and this effect, the factor for which is also not clearly known, is presumed to result
because the graft polymer A of the present invention has high affinity for the particle
and at the same time, is low in the affinity for the back surface before the image
is formed.
[0097] By combining the above-described ink composition, inkjet recording apparatus and
replenishment of ink composition, an image-recorded material free from bleeding of
ink and having high image density and high image quality can be stably obtained over
a long period of time.
Examples
[0098] The present invention is described in greater detail below by referring to Examples,
however, the present invention is not limited thereto.
[Example 1]
<Materials Used>
[0099] In Example 1, the following materials were used. Cyan pigment (coloring material):
phthalocyanine pigment C.I. Pigment Blue (15:3) (Lionol Blue FG-7350, produced by
Toyo Ink Mfg. Co., Ltd.)
Covering agent: [AP-1]
Dispersant: [BZ-2]
Charge-control agent: [CT-1]
Dispersion medium: Isoper G (produced by Exxon Corp.)
[0100] The structures of Covering Agent [AP-1], Dispersant [BZ-2] and Charge-Control Agent
[CT-1] are shown below.

[0101] Covering Agent [AP-1] was obtained by radical-polymerizing styrene, 4-methylstyrene,
butyl acrylate, dodecyl methacrylate and 2-(N,N-dimethylamino)ethyl methacrylate by
using a known polymerization initiator, and further reacting the obtained polymer
with methyl tosylate. The weight average molecular weight was 15,000, the polydispersion
degree (weight average molecular weight/number average molecular weight) was 2.7,
the glass transition point (midpoint) was 51°C, and the softening point as measured
by a strain gauge method was 46°C.
[0102] Dispersant [BZ-2] was obtained by radical-polymerizing stearyl methacrylate in the
presence of 2-mercaptoethanol, reacting the obtained polymer with methacrylic acid
anhydride to obtain Polymer [BP-1] (weight average molecular weight: 7,600) of stearyl
methacrylate, having a methacryloyl group at the terminal, and then radical-polymerizing
this polymer with styrene. The weight average molecular weight was 110,000.
[0103] Polymer [BP-1] dissolved in an amount of 5 g or more per 100 g of Isoper G and Graft
Polymer Dispersant [BZ-2] dissolved in an amount of 5 g or more per 100 g of Isoper
G, though the solution became white turbid.
[0104] Charge-Control Agent [CT-1] was obtained by reacting 1-hexadecylamine with a copolymer
of 1-octadecene and maleic anhydride. The weight average molecular weight was 17,000.
<Preparation of Ink Composition [EC-1]>
[0105] A cyan pigment (10 g) and 20 g of Covering Agent [AP-1] were charged into a desktop
kneader PBV-0.1 manufactured by Irie Shokai K.K. and mixed under heat for 2 hours
by setting the heater temperature to 100°C. Then, 30 g of the obtained mixture was
coarsely ground by Trio Blender manufactured by Trio Science K.K. and further finely
ground by Model SK-M10 Sample Mill manufactured by Kyoritsu Riko K.K. Thereafter,
30 g of the resultant finely ground material was pre-dispersed together with 7.5 g
of Dispersant [BZ-2], 75 g of Isoper G and glass beads having a diameter of about
3.0 mm in a paint shaker manufactured by Toyo Seiki Seisaku-Sho, Ltd. After removing
glass beads, the pre-dispersion was dispersed (formed into particles) together with
zirconia ceramic beads having a diameter of about 0.6 mm in Type KDL Dynomill manufactured
by Shinmaru Enterprise K.K. at a rotation number of 2,000 rpm for 5 hours while keeping
the inner temperature at 25°C and for another 5 hours at 45°C. After removing zirconia
ceramic beads, 316 g of Isoper G and 0.6 g of Charge-Control Agent [CT-1] were added
to the resultant dispersion to obtain Ink Composition [EC-1].
[0106] The physical values of Ink Composition [EC-1] were as follows.
(A) The electric conductivity at 20°C of the ink composition was measured at an applied
voltage of 5 V and a frequency of 1 kHz by using an LCR meter (AG-4311, manufactured
by Ando Electric Co., Ltd.) and an electrode for liquid (Model LP-05, manufactured
by Kawaguchi Electric Works Co., Ltd.) and found to be 100 nS/m.
(B) The ink composition was centrifuged at a rotation number of 14,500 rpm and a temperature
of 20°C for 30 minutes by using a compact high-speed cooling centrifuge (SRX-201,
manufactured by Tomy Seiko Co., Ltd.) to precipitate charged particles and the electric
conductivity of the supernatant was measured and found to be 30 nS/m. The electric
conductivity of the charged particle was 70 nS/m and this was 70% of the electric
conductivity of the ink composition. The electric charge of the charged particle was
positive.
(C) The volume average diameter of charged particles was measured at a rotation number
of 5,000 rpm by using CAPA-700 manufactured by Horiba Ltd. and found to be 0.9 µm.
The number average diameter was 0.15 µm.
(D) The viscosity at 20°C of the ink composition was measured by an E-type viscometer
manufactured by Tokyo Keiki Co., Ltd., using Cornplate type rotor at 100 rpm, and
found to be 1.5 mPa·s. Furthermore, the surface tension at 20°C of the ink composition
was measured by using a FACE automatic surface tension meter manufactured by Kyowa
Interface Science Co., Ltd. and found to be 25 mN/m.
[0107] Ink Composition [EM-1] was obtained in the same manner as in the preparation of Ink
Composition [EC-1] except that a magenta pigment C.I. Pigment Red 57:1 (Brilliant
Carmine 6B, L.R 6B FG-4213, produced by Toyo Ink Mfg. Co., Ltd.) was used in place
of the cyan pigment. Ink Composition [EY-1] was also obtained in the same manner except
that a yellow pigment C.I. Pigment Yellow 180 (Toner Y HG, produced by Clariant K.K.)
was used in place of the cyan pigment. Furthermore, Ink Composition [EB-1] was obtained
in the same manner except that a black pigment C.I. Pigment Black 7 (Carbon Black
MA-8, produced by Mitsubishi Chemical Corporation) was used in place of the cyan pigment.
The physical values of each ink composition obtained are shown in Table 1. These are
all in respective ranges of physical values specified in claim 1 of the present invention.
TABLE 1
Ink Composition |
[EC-1] |
[EM-1] |
[EY-1] |
[EB-1] |
(A) Electric conductivity of ink composition |
100 nS/m |
110 nS/m |
130 nS/m |
80 nS/m |
(B) Ratio of electric conductivity of charged particle |
70% (positive charge) |
75% (positive charge) |
80% (positive charge) |
65% (positive charge) |
(C) Volume average diameter of charged particles |
0.9 µm |
0.7 µm |
1.1 µm |
1.2 µm |
(D) Viscosity of ink composition |
1.5 mPa·s |
1.6 mPa·s |
1.4 mPa·s |
1.8 mPa·s |
<Inkjet Recording>
[0108] Ink Compositions [EC-1], [EM-1], [EY-1] and [EB-1] for four colors were filled into
ink tanks connected to four heads, respectively, of the inkjet recording apparatus
shown in Figs. 1 to 3. The ejection head used here was a 150-dpi (stagger arrangement
in three arrays with a channel density of 50 dpi) 833-channel head of the type shown
in Fig. 2 and the fixing means used was a silicon rubber-made heat roller self-containing
a heater of 1 kW. As the ink temperature-controlling means, an immersion heater and
a stirring blade were provided in the ink tank, the ink temperature was set to 30°C
and the temperature was controlled by a thermostat while rotating the stirring blade
at 30 rpm. The stirring blade was used here to serve also as the stirring means for
preventing precipitation and aggregation. A part of the ink flow path was made transparent,
and an LED light-emitting device and a light-detecting device were disposed to sandwich
the transparent portion. Based on the output signal therefrom, the concentration was
controlled by charging a diluting solution (Isoper G) for ink or a concentrated ink
(prepared by adjusting the solid concentration of the ink composition prepared above
to a 2-fold concentration). The recording medium used was a slightly coated paper
sheet for offset printing. After removing dusts on the surface of the recording medium
by air pump suction, the ejection head was approximated to the recording medium until
the image-forming position. Then, the image data to be recorded were transmitted to
the image data arithmetic and control part and the ink composition was ejected by
sequentially moving the ejection head while conveying the recording medium by the
rotation of the conveyance belt,' thereby forming an image with an image drawing resolution
of 2,400 dpi. The conveyance belt used here was obtained by laminating a metal belt
and a polyimide film. In the vicinity of one edge of this belt, a linear marker was
disposed along the conveyance direction and while optically reading this marker by
the conveyance belt position-detecting means and driving the position-controlling
means, the image was formed. At this time, the distance between the ejection head
and the recording medium was kept to 0.5 mm according to the output from an optical
gap-detecting device. The surface potential of the recording medium at the ejection
was set to -1.5 kV. In performing the ejection, a pulse voltage of +500 V was applied
(pulse width: 50 µsec) and the image was formed at a driving frequency of 15 kHz.
[0109] Immediately after the recording, the image was fixed by using a heat roller. At the
fixing, the temperature of the coated paper was 90°C and the contacting time with
the heat roller was 0.3 seconds.
[0110] On the image-recorded material (printed matter) obtained, streaked unevenness or
bleeding of ink was not observed and a very clear image was formed. During the recording,
image formation failure and the like were not generated at all and the image was completely
free from deterioration due to change in the dot size or the like even when the ambient
temperature was changed or the recording time was increased. Thus, good image formation
could be performed. After the completion of recording, the inkjet recording apparatus
was retreated to 50 mm from the position close to the conveyance belt so as to protect
the inkjet head.
[0111] Also, after the completion of recording, the head was cleaned by supplying Isoper
G in place of ink for 10 minutes and then, the head was housed in a cover filled with
vapor of Isoper G, as a result, a good image-recorded material could be produced for
1 month without requiring a maintenance operation. Furthermore, even when 100 sheets
of the image-recorded materials obtained were superposed and stored at 40°C for 3
days, the image-recorded materials did not adhere to each other.
[Comparative Example 1]
[0112] Ink Composition [RC-1] was prepared in the same manner as in Example 1 except that
in the preparation of Ink Composition [EC-1] of Example 1, Charge-Control Agent [CT-1]
was not added. The physical values of the obtained [RC-1] are shown in Table 2. The
value in (A) electric conductivity of ink composition was small.
[0113] When inkjet recording was performed using Ink Composition [RC-1] and using the same
apparatus as in Example 1, an ink droplet was not ejected.
[Comparative Example 2]
[0114] Ink Composition [RC-2] was prepared in the same manner as in Example 1 except that
in the preparation of Ink Composition [EC-1] of Example 1, 1.5 g of Charge-Control
Agent [CT-1] was added. The physical values of the obtained [RC-2] are shown in Table
2. The value in (B) ratio of electric conductivity of charged particle to the ink
composition was small.
[0115] When inkjet recording was performed using Ink Composition [RC-2] and using the same
apparatus as in Example 1, an ink droplet was ejected but bleeding of ink was observed
on the obtained image.
[Comparative Example 3]
[0116] Ink Composition [RC-3] was prepared in the same manner as in Example 1 except that
in the preparation of Ink Composition [EC-1] of Example 1, [BZ-2] was used in place
of Covering Agent [AP-1] ([BZ-2] was used also as the dispersant and used in a total
amount of 27.5 g). The physical values of the obtained [RC-3] are shown in Table 2.
The value in (C) volume average diameter of charged particles was small.
[0117] When inkjet recording was performed using Ink Composition [RC-3] and using the same
apparatus as in Example 1, an ink droplet was ejected but bleeding of ink was observed
on the obtained image.
[Comparative Example 4]
[0118] Ink Composition [EC-1] of Example 1 was concentrated under pressure to a solid concentration
of 35% and this was designated as Ink Composition [RC-4]. The physical values of the
obtained [RC-4] are shown in Table 2. The value in (D) viscosity at 25°C of ink composition
was large.
[0119] When inkjet recording was performed using Ink Composition [RC-C] and using the same
apparatus as in Example 1, an ink droplet was not ejected.
TABLE 2
Comparative Example No. |
comparative Example 1 |
Comparative Example 2 |
Comparative Example 3 |
Comparative Example 4 |
Ink Composition |
[RC-1] |
[RC-2] |
[RC-3] |
[RC-4] |
(A) Electric conductivity of ink composition |
2 nS/m |
180 nS/m |
80 nS/m |
260 nS/m |
(B) Ratio of electric conductivity of charged particle |
75% (positive charge) |
40% (positive charge) |
70% (positive charge) |
70% (positive charge) |
(C) Volume average diameter of charged particles |
0.9 µm |
0.9 µm |
0.15 µm |
0.9 µm |
(D) Viscosity of ink composition |
1.4 mPa·s |
1.9 mPa·s |
2.2 mPa·s |
15 mPa·s |
Ejection property of ink droplet |
not ejected |
ejected but image is blurred |
ejected but image is blurred |
not ejected |
[Comparative Example 5]
[0120] Inkjet recording was performed by using Ink Composition [EC-1] of Example 1 in the
same manner as in Example 1 except that a concentrated ink was not charged. Until
the third day, an ink droplet was ejected and a good image-recorded material could
be produced, but on the fourth day, bleeding of ink was observed on the image. The
ink composition was recovered and the volume average diameter of charged particles
was measured and found to be 0.18 µm. The reason why the volume average diameter was
decreased is not clearly known but is considered because charged particles having
a large diameter were preferentially concentrated and ejected and charged particles
having a small diameter were remaining in the ink composition.
[Example 2]
<Materials Used>
[0121] In Example 2, [AP-2] was used as the covering agent in place of [AP-1]. Covering
Agent [AP-2] was obtained by dehydration-condensing adipic acid with a slightly excessive
amount of p-xylene glycol to synthesize a polyester having a hydroxyl group at both
terminals and then reacting the polyester with hexamethylene diisocyanate. The weight
average molecular weight was 20,000, the polydispersion degree (weight average molecular
weight/number average molecular weight) was 1.6, the glass transition point (midpoint)
was -13°C, the melting point was 75°C and the softening point was 74°C. Also, tetrabutylammonium
p-toluenesulfonate obtained from tetrabutylammonium hydroxide and p-toluenesulfonic
acid was used. Except for these, the same materials as used in Example 1 were used.

<Preparation of Ink Composition [EC-2]>
[0122] A cyan pigment (10 g), 20 g of Covering Agent [AP-2] and 1 g of tetrabutylammonium
p-toluenesulfonate were charged into a flask and mixed under heat for 1 hour while
heating the flask by a heater such that the inner temperature became 110°C. Then,
31 g of the obtained mixture was coarsely ground by Trio Blender manufactured by Trio
Science K.K. and further finely ground by Model SK-M10 Sample Mill manufactured by
Kyoritsu Riko K.K. Thereafter, 31 g of the resultant finely ground material was pre-dispersed
together with 9 g of Dispersant [BZ-2], 75 g of Isoper G and glass beads having a
diameter of about 3.0 mm in a paint shaker manufactured by Toyo Seiki Seisaku-Sho,
Ltd. After removing glass beads, the pre-dispersion was dispersed (formed into particles)
together with zirconia ceramic beads having a diameter of about 0.6 mm in Type KDL
Dynomill manufactured by Shinmaru Enterprise K.K. at a rotation number of 2,000 rpm
for 5 hours while keeping the inner temperature at 25°C and for another 5 hours at
50°C. After removing zirconia ceramic beads, 360 g of Isoper G and 0.4 g of Charge-Control
Agent [CT-1] were added to the resultant dispersion to obtain Ink Composition [EC-2].
[0123] Ink Composition [EC-2] was measured in the same manner as in Example 1 and found
to have the following physical values.
(A) The electric conductivity at 20°C of the ink composition was 120 nS/m.
(B) The ratio of the electric conductivity of the charged particle to the electric
conductivity of the ink composition was 85%. The electric charge of the charged particle
was positive.
(C) The volume average diameter of charged particles was 1.0 µm.
(D) The viscosity at 20°C of the ink composition was 1.9 mPa·s and the surface tension
at 20°C of the ink composition was 23 mN/m.
<Inkjet Recording>
[0124] Using Ink Composition [EC-2], inkjet recording was performed for 1 month in the same
manner as in Example 1 while charging a concentrated ink. During this recording, a
very clear image free from streaked unevenness or bleeding of ink was obtained and
a good image-recorded material could be produced without requiring a maintenance operation.
[Comparative Example 6]
[0125] Ink Composition [RC-5] was prepared in the same manner as in Example 1 except that
in the preparation of Ink Composition [EC-1] of Example 1, Polymer [BP-1] obtained
in the process of synthesizing [BZ-2] was used in place of Dispersant [BZ-2]. The
volume average diameter of charged particles was 2.0 µm.
[0126] Ink Composition [EC-5] was measured in the same manner as in Example 1 and found
to have the following physical values.
(A) The electric conductivity at 20°C of the ink composition was 80 nS/m.
(B) The ratio of the electric conductivity of the charged particle to the electric
conductivity of the ink composition was 40%. The electric charge of the charged particle
was positive.
(C) The volume average diameter of charged particles was 2.0 µm.
(D) The viscosity at 20°C of the ink composition was 5.5 mPa·s and the surface tension
at 20°C of the ink composition was 30 mN/m.
[0127] When inkjet recording was performed using Ink Composition [RC-5] and using the same
apparatus as in Example 1, the ejection of an ink droplet was stopped after 2 weeks.
The head was disassembled and inspected, as a result, ink particles were attaching
to the ejection port to cause clogging of the ejection port. Also, when 100 sheets
of the image-recorded materials obtained were superposed and stored at 40°C for 3
days, the image-recorded materials adhered to each other.
[Comparative Example 7]
[0128] Ink Composition [RC-6] was prepared in the same manner as in Example 1 except that
in the preparation of Ink Composition [EC-1] of Example 1, a non-graft polymer [BP-2]
(weight average molecular weight: 130,000) obtained by directly polymerizing stearyl
methacrylate and styrene was used in place of Dispersant [BZ-2]. The volume average
diameter of charged particles was 2.8 µm.
[0129] Polymer [BP-2] dissolved only in an amount of 3 g or less per 100 g of Isoper G.
[0130] Ink Composition [EC-6] was measured in the same manner as in Example 1 and found
to have the following physical values.
(A) The electric conductivity at 20°C of the ink composition was 8 nS/m.
(B) The ratio of the electric conductivity of the charged particle to the electric
conductivity of the ink composition was 30%. The electric charge of the charged particle
was positive.
(C) The volume average diameter of charged particles was 2.8 µm.
(D) The viscosity at 20°C of the ink composition was 3.0 mPa·s and the surface tension
at 20°C of the ink composition was 28 mN/m.
[0131] When inkjet recording was performed using Ink Composition [RC-6] and using the same
apparatus as in Example 1, the ejection of an ink droplet was stopped after 1 week.
The head was disassembled and inspected, as a result, ink particles were attaching
to the ejection port to cause clogging of the ejection port.
[Comparative Example 8]
[0132] An attempt was made to prepare an ink composition in the same manner as in Example
1 except that in the preparation of Ink Composition [EC-1] of Example 1, a non-graft
polymer [BP-3] (weight average molecular weight: 70,000) obtained by polymerizing
only styrene was used in place of Dispersant [BZ-2], but particles were not formed.
[0133] Polymer [BP-3] was a polymer constituted only by the main chain part of Dispersant
[BZ-2] but dissolved only in an amount of 3 g or less per 100 g of Isoper G.
[Example 3]
<Materials Used>
[0134] In Example 3, the following materials were used. Magenta pigment (coloring material):
monoazo lake pigment C.I. Pigment Red (57:1) (Symuler
Brilliant Carmine 6B229, produced by Dai-Nippon Ink & Chemicals, Inc.)
Covering agent: [AP-2] used in Example 2
Dispersant: [BZ-7]
Tetrabutylammonium p-toluenesulfonate
Charge-control agent: [CT-1]
Dispersion medium: Isoper G (produced by Exxon Corp.)
[0135] The structures of Covering Agent [AP-2] and Dispersant [BZ-7] are shown below.

[0136] Dispersant [BZ-7] was obtained by radical-polymerizing butyl methacrylate, 2-methoxyethyl
methacrylate and Polymer [BP-1] of stearyl methacrylate, having a methacryloyl group
at the terminal, which was synthesized in Example 1. The weight average molecular
weight was 180,000.
[0137] Dispersant [BZ-7] dissolved in an amount of 5 g or more per 100 g of Isoper G, though
the solution became white turbid.
<Preparation of Ink Composition [EM-2]>
[0138] A magenta pigment (10 g), 20 g of Covering Agent [AP-2] and 1 g of tetrabutylammonium
p-toluenesulfonate were charged into a flask and mixed with stirring under heat for
1 hour while heating the flask by a heater such that the inner temperature became
110°C. Then, 31 g of the obtained mixture was coarsely ground by Trio Blender manufactured
by Trio Science K.K. and further finely ground by Model SK-M10 Sample Mill manufactured
by Kyoritsu Riko K.K. Thereafter, 31 g of the resultant finely ground material was
pre-dispersed together with 9 g of Dispersant [BZ-7], 75 g of Isoper G and glass beads
having a diameter of about 3.0 mm in a paint shaker manufactured by Toyo Seiki Seisaku-Sho,
Ltd. After removing glass beads, the pre-dispersion was dispersed (formed into particles)
together with zirconia ceramic beads having a diameter of about 0.6 mm in Type KDL
Dynomill manufactured by Shinmaru Enterprise K.K. at a rotation number of 3,000 rpm
for 8 hours while keeping the inner temperature at 40°C. After removing zirconia ceramic
beads, 360 g of Isoper G and 0.4 g of Charge-Control Agent [CT-1] were added to the
resultant dispersion to obtain Ink Composition [EM-2]. The volume average diameter
of charged particles was 0.7 µm.
[0139] Ink Composition [EM-2] was measured in the same manner as in Example 1 and found
to have the following physical values.
(A) The electric conductivity at 20°C of the ink composition was 120 nS/m.
(B) The ratio of the electric conductivity of the charged particle to the electric
conductivity of the ink composition was 54%. The electric charge of the charged particle
was positive.
(C) The volume average diameter of charged particles was 0.7 µm.
(D) The viscosity at 20°C of the ink composition was 1.7 mPa·s and the surface tension
at 20°C of the ink composition was 26 mN/m.
<Inkjet Recording>
[0140] Using Ink Composition [EM-2], inkjet recording was performed for 1 month in the same
manner as in Example 1 while charging a concentrated ink. During this recording, a
very clear image free from streaked unevenness or bleeding of ink was obtained and
a good image-recorded material could be produced without requiring a maintenance operation.
[Comparative Example 9]
[0141] Ink Composition [RM-1] was prepared in the same manner as in Example 2 except that
in the preparation of Ink Composition [EM-2] of Example 3, a non-graft polymer [BP-4]
(weight average molecular weight: 180,000) obtained by directly polymerizing stearyl
methacrylate, butyl methacrylate and 2-methoxyethyl methacrylate was used in place
of Dispersant [BZ-7]. The volume average diameter of charged particles was 2.4 µm.
[0142] Polymer [BP-4] dissolved only in an amount of 3 g or less per 100 g of Isoper G.
[0143] Ink Composition [RM-1] was measured in the same manner as in Example 1 and found
to have the following physical values.
(A) The electric conductivity at 20°C of the ink composition was 9 nS/m.
(B) The ratio of the electric conductivity of the charged particle to the electric
conductivity of the ink composition was 36%. The electric charge of the charged particle
was positive.
(C) The volume average diameter of charged particles was 2.4 µm.
(D) The viscosity at 20°C of the ink composition was 4.0 mPa·s and the surface tension
at 20°C of the ink composition was 29 mN/m.
[0144] When inkjet recording was performed using Ink Composition [RM-1] and using the same
apparatus as in Example 1, the ejection of an ink droplet was stopped after 2 weeks.
The head was disassembled and inspected, as a result, ink particles were attaching
to the ejection port to cause clogging of the ejection port.
[Comparative-Example 10]
[0145] An attempt was made to prepare an ink composition in the same manner as in Example
2 except that in the preparation of Ink Composition [EM-1] of Example 3, a non-graft
polymer [BP-5] (weight average molecular weight: 110,000) obtained by polymerizing
butyl methacrylate and 2-methoxyethyl methacrylate was used in place of Dispersant
[BZ-7], but particles were not formed.
[0146] Polymer [BP-5] was a polymer constituted only by the main chain part of Dispersant
[BZ-7] but dissolved only in an amount of 3 g or less per 100 g of Isoper G.
[Example 4]
<Materials Used>
[0147] In Example 4, the following materials were used. Yellow pigment (coloring material):
disazo pigment C.I. Pigment Yellow 180 (Toner Y HG, produced by Clariant K.K.)
Covering agent: [AP-1]
Dispersant: [BZ-8]
Charge-control agent: [CT-1]
Dispersion medium: Isoper G (produced by Exxon Corp.)
[0148] The structure of Dispersant [BZ-8] is shown below.

[0149] Dispersant [BZ-8] was obtained by polymerizing methyl methacrylate, 2-hydroxyethyl
acrylate and methacryloxypropyl-terminated polydimethylsiloxane DMS-R18 (molecular
weight: 4,500 to 5,500) produced by Chisso Corporation. The weight average molecular
weight was 60,000.
[0150] DMS-R18 and Dispersant [BZ-8] each dissolved in an amount of 5 g or more per 100
g of Isoper G.
<Preparation of Ink Composition [EY-1]>
[0151] A yellow pigment (8 g) and 22 g of Covering Agent [AP-1] were charged into a desktop
kneader PBV-0.1 manufactured by Irie Shokai K.K. and mixed under heat for 2 hours
by setting the heater temperature to 100°C. Then, 30 g of the obtained mixture was
coarsely ground by Trio Blender manufactured by Trio Science K.K. and further finely
ground by Model SK-M10 Sample Mill manufactured by Kyoritsu Riko K.K. Thereafter,
30 g of the resultant finely ground material was pre-dispersed together with 7.5 g
of Dispersant [BZ-8], 75 g of Isoper G and glass beads having a diameter of about
3.0 mm in a paint shaker manufactured by Toyo Seiki Seisaku-Sho, Ltd. After removing
glass beads, the pre-dispersion was dispersed (formed into particles) together with
zirconia ceramic beads having a diameter of about 0.6 mm in Type KDL Dynomill manufactured
by Shinmaru Enterprise K.K. at a rotation number of 3,000 rpm for 8 hours while keeping
the inner temperature at 45°C. After removing zirconia ceramic beads, 316 g of Isoper
G and 0.6 g of Charge-Control Agent [CT-1] were added to the resultant dispersion
to obtain Ink Composition [EY-1]. The volume average diameter of charged particles
was 1.2 µm.
[0152] Ink Composition [EY-1] was measured in the same manner as in Example 1 and found
to have the following physical values.
(A) The electric conductivity at 20°C of the ink composition was 90 nS/m.
(B) The ratio of the electric conductivity of the charged particle to the electric
conductivity of the ink composition was 73%. The electric charge of the charged particle
was positive.
(C) The volume average diameter of charged particles was 1.2 µm.
(D) The viscosity at 20°C of the ink composition was 1.3 mPa·s and the surface tension
at 20°C of the ink composition was 20 mN/m.
<Inkjet Recording>
[0153] Using Ink Composition [EY-1], inkjet recording was performed for 1 month in the same
manner as in Example 1 while charging a concentrated ink. During this recording, a
very clear image free from streaked unevenness or bleeding of ink was obtained and
a good image-recorded material could be produced without requiring a maintenance operation.
[Comparative Example 11]
[0154] An attempt was made to prepare an ink composition in the same manner as in Example
3 except that in the preparation of Ink Composition [EY-1] of Example 4, methacryloxypropyl-terminated
polydimethylsiloxane DMS-R18 produced by Chisso Corporation was used in place of Dispersant
[BZ-8], but particles were not formed.
[Comparative Example 12]
[0155] An attempt was made to prepare an ink composition in the same manner as in Example
4 except that in the preparation of Ink Composition [EY-1] of Example 4, a non-graft
polymer [BP-6] (weight average molecular weight: 140,000) obtained by polymerizing
methyl methacrylate and 2-hydroxyethyl acrylate was used in place of Dispersant [BZ-8],
but particles were not formed.
[0156] Polymer [BP-6] was a polymer constituted only the main chain part of Dispersant [BZ-8]
but dissolved only in an amount of 3 g or less per 100 g of Isoper G.
[0157] The present invention can ensure that an ink droplet can be always stably ejected
in inkjet recording over a long period of time and a high-quality image can be formed
for a long period of time, by seting the electric conductivity and the viscosity of
the composition, and the electric conductivity of the charged particle to the specified
range.
[0158] This application is based on Japanese patent application JP 2003-122111, filed on
April 25, 2003 and Japanese patent application JP 2003-154781, filed on May 30, 2003,
the entire content of which is hereby incorporated by reference, the same as if set
forth at length.